The natural gas industry has long sought a way in the presence of excess oxygen of removing oxides of nitrogen from flue gas, which may typically be at 375-425 K and may contain 13-14 volume % water vapor, 5-6% oxygen, 6-7% carbon dioxide and 100 ppm nitric oxide. Oxides of nitrogen can also arise in flue gas in other industries where combustion occurs and in petrol motor car exhausts. It is desirable to convert these compounds into harmless compounds before they are released into the atmosphere.
EP-B-256590 discloses a titanium/vanadium catalyst on a silica carrier for reducing nitrogen oxides in the presence of ammonia to nitrogen and water. This is stated in the EP-B to be only 64% effective at 423 K even at a gas hourly space velocity (explained later) of 10000, which is too low a throughput for some large-scale industries.
Further catalysts for this reaction are disclosed in Catalysis Today 7 (1990) 157-165, (copper salts on carbon supports), Ind Eng Chem Prod Res Dev 20 (1981) 301-304 (molybdenum on titania/zirconia/alumina/silica, International Chemical Engineering 15 (July 1975) 546-549 (Pt, CuO--Cr.sub.2 O.sub.3, CuO, V.sub.2 O.sub.5, NiO, Fe.sub.2 O.sub.3, MnO--Cr.sub.2 O.sub.3, MoO.sub.3, Co.sub.3 O.sub.4, MnO.sub.2, Cr.sub.2 O.sub.3 and WO.sub.3), and Bulletin of Chemical Society of Japan 54 (1981) 3347-3350 (Mn.sub.2 O.sub.3 and Mn.sub.2 O.sub.3 --Co.sub.3 O.sub.4 in 1 mm particles from calcined carbonate-precipitated nitrate solution), the last-named suffering from slow partially irreversible poisoning by sulphur dioxide.
Wakker and Gerritsen, Proc American Chemical Society, Division of Fuel Chemistry 35(1) (1990) 170 discloses a catalyst of MnO supported on gamma-alumina, the MnO being obtained by impregnating the alumina in manganese acetate, sulphate, nitrate or oxalate solution, drying, optionally calcining, and reducing with hydrogen at 600.degree. C. This catalyst can remove H.sub.2 S from hot reactor gases. However, it cannot reduce oxides of nitrogen.
A similar catalyst (manganese acetate or nitrate solution impregnated into steam-activated carbonised peat and calcined in 2% oxygen) was disclosed for reducing NO with ammonia at the International Symposium on Carbon, TsuKuba, 1990, page 590 of the Extended Abstracts. This catalyst can successfully reduce NO below 425 K but its activity decreases irreversibly above that temperature. This is a serious problem since it cannot be guaranteed that the temperature of a flue in an industrial process (such as petrochemical cracking) will not accidentally rise above 425 K.